In the present article we summarize our results concerning the in situ ultrastructural organization of the nuclear steps of mRNA production using several cellular models including normal noninfected cells, herpes simplex virus type 1 and adenovirus type 5 infected cells, and transiently transfected cells. We confirm that perichromatin fibrils are the in situ morphological expression of nascent transcripts and the main support of splicing. Special emphasis is devoted to the clusters of interchromatin granules which seem to be involved in several central functions including accumulations of snRNP components as well as sorting and/or regulation of export of different RNA species.